Cationic paint binders which can be employed according to the present invention, especially for the formulation of cathodically depositable electrocoating paints, crosslink to a significant extent at elevated temperature by transesterification, transurethanization, or by the reaction of terminal double bonds. It is known that crosslinking reactions of this type are catalyzed by metal compounds. In the curing of cathodically deposited paint films, such a catalysis is necessary virtually in all cases in order to attain the spectrum of properties required by industrial users.
The most important catalysts presently used in industry are organic compounds of tetravalent tin which are employed, for example, as dibutyltin oxide (C.sub.4 H.sub.9).sub.2 SnO, as dibutyltin dilaurate (C.sub.4 H.sub.9).sub.2 Sn (OCOC.sub.12 H.sub.25).sub.2 or as reaction products of dibutyltin oxide with alcohols or phenols. However, all of these catalysts have disadvantages.
Thus, dibutyltin oxide is a solid which has to be incorporated in the paint batch in as finely divided a form as possible, since it is fully effective as a catalyst only in solution. Longer homogenization times are therefore necessary when this catalyst is employed. If the comminution is effected by grinding in a paint binder, as is frequently the custom with pigments and extenders, then the catalytic effect of the dibutyltin oxide can be set off or initiated by the temperature rise occurring in the grinding process, and the reactivity of the binder can be prematurely activated to an undesirable degree. On the other hand, the use of dispersion media which are inert toward the binders gives rise to non-crosslinked molecular segments in the stoved film and is likewise unsuccessful. When dibutyltin dilaurate is used, acid is liberated by hydrolysis which can cause interference, especially in the operation of electrocoating plants. Dibutyltin oxide and dibutyltin dilaurate are also often not fully compatible with the paint binders used. In many cases this impairs storage stability, especially that of dilute paints used as a topping-up material in electrocoating plants.
The reaction products of dibutyltin oxide (DBTO) with alcohols or phenols, as described in EP 0,261,486 B1, possess in the ready-for-use paint excellent compatibility and stability. However, at the temperatures necessary for the crosslinking of the deposited films, decomposition products form during the crosslinking process which cause surface defects in the stoved paint film, such as pinholing and cratering.
It has now been found that an industrially advantageous catalysis for the crosslinking reactions of cathodically depositable paint binders for electrocoating paints is possible even when using dibutyltin oxide, if the mechanical dispersion of the dibutyltin oxide is effected in a mixture of at least one oxazolidine compound and specific solvents.